Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase involved in many essential cellular processes. The core enzyme is a heterotrimer in which a catalytic subunit (PP2Ac) associates with a scaffold subunit A and with one of a diverse set of regulatory subunits B; the catalytic subunit must be activated for proper holoenzyme formation. Guo et al. have determined the structure of PP2Ac in complex with a fusion protein comprising the PP2A phosphatase activator protein (PTPA) and a mini-A subunit, and with ATPgS bound. The structure shows that both PTPA and PP2Ac interact with ATPgS and that the phosphate of ATP binds to catalytic metal ions. The authors suggest that the ATP-binding pocket is created upon complex formation and that this results in ATP hydrolysis at the PP2Ac active site, which is also where the removal of phosphates from serine/threonine occurs. Biochemical experiments suggest that ATP facilitates binding of the appropriate metal ions to PP2A and that this reduces nonspecific phosphatase removal. Coordinated regulation of both PP2Ac activity and holoenzyme formation would allow for a tight regulation of PP2A function.